1. Field of the Invention
The invention relates to an air-conditioning system for preparing pressurized air for the air-conditioning of a space, in particular for the air-conditioning of airplane cabins, and a corresponding process.
2. Prior Art
The fresh air for the air-conditioning of airplane cabins is usually prepared from the air tapped under high pressure and at high temperature from the power unit, referred to as bleed. The air-conditioning systems draw the cooling capacity required for the preparation from the pressure and temperature potential of the precompressed power unit air. The bleed is cooled off during the course of the fresh air preparation process and its pressure drops to the cabin pressure of 1 bar in ground operation or roughly 0.8 bar in flight. During the process of fresh air preparation the air is also dehumidified in order to prevent an icing of individual components of the air-conditioning system and ice crystal formation in the fresh air to be prepared. Dehumidification is necessary primarily in ground operation, however, because in flight, i.e., at high altitudes, the ambient air and thus the tapped power unit air is extremely dry anyway.
With the help of FIG. 4, an air-conditioning system is described below of the kind used in today""s Airbus and Boeing passenger aircraft, for example the A330/340 and Boe 757/767.
Via a flow control valve FCV, that bleed quantity is drawn from a power unit and fed to the system at 1.5 to 3.5 bar and 150xc2x0 C. to 230xc2x0 C. that is needed to supply the cabin with fresh air. In ground operation the bleed is drawn from an auxiliary power unit and fed to the system at roughly 3 bar. The bleed is first guided through a primary heat-exchanger PHX and cooled off to approx. 100xc2x0 C.
The bleed is then further compressed in a compressor C to approx. 4.5 bar and 160xc2x0 C. and cooled off again in a main heat-exchanger MHX to approx. 45xc2x0 C. The high pressure of 4.5 bar is necessary to be able to achieve a high degree of dehumidification in the subsequent water separation circuit. This system is therefore also known as a xe2x80x9chigh pressure water separation circuitxe2x80x9d.
The high pressure water separation circuit comprises a condenser CON as is proposed in EP 0 019 493 A3, and a water extractor WE downstream from the condenser CON. The compressed, cooled bleed is cooled off by roughly T=xe2x88x9215 K in the condenser CON, the condensed water is then extracted in the water extractor WE, and the pressure of the air dehumidified in this way is then expanded in a turbine T to the cabin pressure of roughly 1 bar, while the temperature at the turbine output is roughly xe2x88x9230xc2x0 C. Before it is mixed as fresh air in a mixing chamber with recycled cabin air, the thus prepared bleed is guided in heat-exchanging manner through the condenser CON of the high pressure water extraction circuit in order to cool the compressed, cooled bleed to down to the temperature necessary for water extraction in the water extractor WE. In this connection, the pressure-relieved, cooled air in the turbine T heats up again by the equivalent of xcex94T=+15 K to roughly xe2x88x9215xc2x0 C.
The air-conditioned air is then mixed with recycled cabin air in a mixing chamber not illustrated. The temperature at the turbine output can be raised by means of a temperature control valve TCV in order to obtain an optimal mixing temperature with the recycled cabin air mixed in. For this purpose, a portion of the bleed precooled in the primary heat-exchanger PHX is diverted and guided back to the prepared airflow behind the turbine T.
In addition to the condenser CON, a reheater REH is provided upstream from the condenser CON in the high pressure water extraction circuit. The compressed, cooled bleed is first guided through the reheater REH before it enters the condenser CON, and subsequently the now dehumidified air is again guided through the reheater REH before it enters the turbine T. In this connection, the reheater REH""s task is essentially to heat the dehumidified air by roughly xcex94T=5 K and to evaporate residual moisture from the dehumidified air with simultaneous energy regeneration, before the air enters the turbine. This is because residual moisture in the form of fine droplets can destroy the turbine wheel surfaces and the turbine nozzles, since the air in the turbine T almost reaches the speed of sound. A second function of the reheater REH consists in relieving the condenser CON in that the compressed, cooled bleed is cooled by the equivalent of xcex94T=xe2x88x925 K before entering the condenser CON.
The energy generated in the turbine T is used to drive the compressor C on the one hand and a fan F on the other hand. All three wheels, that is, turbine/compressor/fan, are mounted on a shared shaft and form the air cycle machine, also referred to as three-wheel machine. The fan F conveys a cooling airflow diverted from the ambient airflow through a cooling shaft in which the primary heat-exchanger and the main heat-exchanger PHX, MHX are arranged. The fan F must be actively driven by the turbine T in ground operation in particular. The ram air is sufficient to drive the fan in flight; if necessary it can be controlled by an adjustable valve at the cooling shaft entrance.
The entire system is designed for ground operation at an ambient temperature of 38xc2x0 C. To optimize the efficacy of the heat-exchanging process in the cooling shaft, the water gained in the high pressure water extraction circuit is fed at a temperature of approx. T=25xc2x0 C. and a pressure of 3.5 bar to the cooling shaft entrance for evaporation there, thereby improving the efficacy of the heat-exchanger.
In the event that the air cycle machine ACM is completely unavailable because, for example, the necessary mass pressurized airflow to fulfill the parameters required for the system to function cannot be attained, a bypass valve BPV is provided in order to circumvent the turbine T. In this case, a check valve CV opens automatically in that an overpressure triggering the check valve CV builds up before the compressor C for lack of drive by the turbine T. The compressor C is circumvented or xe2x80x9cshort-circuitedxe2x80x9d by the check valve CV opening. In this condition, the fresh air is immediately fed through the primary and main heat-exchangers PHX, MHX directly to the mixing chamber downstream from the air-conditioning system for mixing with recycled cabin air.
As mentioned in the beginning, ice formation in the prepared fresh air presents a problem. In order to avoid ice formation, an anti-icing valve AIV is provided with which a portion of the air bled from the power unit is immediately diverted and again fed to the prepared airflow behind the turbine T.
A thermodynamically improved variant of this air-conditioning system provides for the air cycle machine ACM to be expanded by a second turbine. In this way, the turbine/compressor/fan three-wheel machine becomes a turbine/turbine/compressor/fan four-wheel machine (see U.S. Pat. No. 5,086,622 to Warner). The second turbine is mounted with the other wheels on a shared shaft in order to feed the energy generated by the turbines back into the air-conditioning system, in the manner of the conventional three-wheel system. The second turbine completes the first turbine in such a way that the pressure of the air dehumidified in the high pressure water extraction circuit is dropped in two steps, in connection with which the condenser of the high pressure water extraction circuit is arranged in heat-exchanging manner with the air conduit between the two turbines.
This saves more energy than the conventional design of the air-conditioning system because the air emerging from the first turbine is comparably warm, preferably above 0xc2x0 C. to avoid ice, and this air is heated in the condenser CON by xcex94T=+15 Kelvin, for example, to a comparably high energy level in such a way that the second turbine can utilize this high energy level to generate energy that is lost with the conventional system. This system is known in technical circles as a xe2x80x9ccondensing cyclexe2x80x9d.
The bleeding of the fresh air to be prepared directly from the power unit proves to be problematic in the above-described air-conditioning systems. This is because the power unit temperature increases as the power unit air throughput decreases, at constant output. Since the power units are already being operated at their highest admissible temperature limit, a bleeding of the air to be prepared from the power unit is obligatorily connected with a reduction of the power unit output.
It was already proposed in the past to suck the necessary fresh air for the air-conditioning of the airplane cabin through a separately driven compressor from the ambient air and to compress it. The drive capacity required for this and the accordingly necessary drive machines are enormous and extremely heavy, however, and this is not compatible with the demands made on an airplane.
It has also been proposed in this connection to suck a portion of the necessary fresh air from the surroundings by means of a compressor driven by the turbine, to compress this air and feed it to the tapped airflow so that as a mixed airflow, its pressure is then dropped in the turbine and it is thereby cooled. However, it is difficult to implement such a system in energy-saving and structurally acceptable manner, particularly in terms of a compact, simple design and with a low weight.
In U.S. Pat. No. 5,299,763 to Bescoby et al, it is proposed to combine the ambient airflow and the tapped airflow in the turbine. This was unable to assert itself, however. In particular, two high pressure water extraction circuits must be produced with corresponding weight disadvantages and complex design. The turbine used for it is also extremely complex because it is divided into two and it is not optimal in terms of efficiency.
The technical problem of the present invention is therefore to propose an air-conditioning system and an air-conditioning process with effective water extraction that is highly efficient, affects the power unit capacity only slightly and avoids the above-mentioned construction-related disadvantages.
This technical problem is solved by an air-conditioning system and an air-conditioning process for preparing air for the air conditioning of an enclosed space in accordance with the features indicated by the following objectives.
An object of the process of the present invention is to provide the steps of tapping of a first, pressurized partial airflow (xe2x80x9ctapped airflowxe2x80x9d) from a power unit or auxiliary power unit, tapping and compressing of a second partial airflow (xe2x80x9cambient airflowxe2x80x9d) from the surroundings, combining the tapped airflow and the compressed ambient airflow to form a mixed airflow, expanding the mixed airflow and conveying the mixed airflow for the air-conditioning of the enclosed space, wherein the tapped airflow is guided in a heat-exchanging manner past the mixed airflow in order to cool the tapped airflow before combining with the ambient airflow.
Another object of the present invention is to have the tapped airflow guided past the mixed airflow before and after the pressure of the mixed airflow has been dropped, and the energy required to compress the ambient air is obtained in the expansion of the mixed airflow and is utilized regeneratively.
A further object of the present invention is to have energy extraneous to the system fed in to compress the ambient airflow and the ratio of mass tapped airflow to mass ambient airflow range between 100:0 and 50:50.
A still further object of the present invention is to have the ratio of mass tapped airflow to mass ambient airflow be 65:35, and the water separated from the mixed airflow before the step of the mixed airflow.
Another object of the present invention is to have water separated from the tapped airflow and/or the compressed ambient airflow before the step of combining them, and to have the ambient airflow compressed to a pressure in the 0.8 bar to 4 bar range during the step of compressing.
It is still another object of the invention to have the tapped airflow made available at an input pressure ranging from 1.5 bar in flight to 4 bar in ground operation, and that the pressure ratios of the tapped airflow and the ambient airflow be selected in such a way that in ground operation, a pressure of roughly 3.4 bar ensues for the mixed airflow when the tapped airflow and the compressed ambient airflow are combined.
Another object of the present invention is to have the tapped airflow guided in a heat-exchanging manner past a separate cooling airflow, and in the process, undergo a relatively greater pressure change of about xcex94p=0.05 bar to about xcex94p=0.3 bar, and that the compressed ambient airflow be guided in a heat-exchanging manner past a separate cooling airflow and in the process, undergo a relatively small pressure change of roughly xcex94p=0.01 bar to xcex94p =0.05 bar.
It is a further object of the present invention to have at first the compressed ambient airflow and then the tapped airflow guided in heat-exchanging manner past the separate cooling airflow, and that water separated from the tapped airflow and/or the mixed airflow and/or the compressed ambient airflow be fed into the separate cooling airflow for evaporation.
A further object of the present invention is to have the expansion of the mixed airflow take place in two steps and between the two expanding steps the mixed airflow is guided in a heat-exchanging manner past the tapped airflow to cool the tapped airflow, and after expanding the mixed airflow, water is removed from the mixed airflow.
A further object of the present invention is that the step of compressing the ambient airflow and the step of the expanding the mixed airflow, each takes place in two steps, in connection with which, for each compression step energy is utilized regeneratively from only one of the two expanding steps in each case.
It is yet another object of the present invention to have an air-conditioning system for preparing pressurized air for the air-conditioning of a space, which system comprises at least one compressor installation (C; C1, C2) that is in contact with ambient air and compresses a partial airflow (xe2x80x9cambient airflowxe2x80x9d) originating from the ambient air, a mixing element (xe2x80x9cXxe2x80x9d) in which the compressed ambient airflow is combined with a pressurized partial airflow (xe2x80x9ctapped airflowxe2x80x9d) tapped from a power unit into a mixed airflow, and at least one expansion installation (T; T1, T2) in which pressure of the mixed airflow is dropped to a lower pressure level and the mixed airflow is cooled off in the process, characterized by at least one heat-exchanger (REH, CON) through which on the one hand the tapped airflow flows and, on the other hand, the mixed airflow flows.
Another object of the invention is that at least one heat-exchanger (REH, CON) includes a (first) heat-exchanger (CON) arranged in such a way that the mixed airflow flows through it after the expanding of the mixed airflow in the expansion installation, and that of the at least one heat-exchanger (REH, CON) includes a (second) heat-exchanger arranged in such a way that the mixed airflow flows through it before the mixed airflow is expanded in the expansion installation.
A still further object of the invention is that the expansion installation (T; T1, T2) has at least one turbine wheel and the compressor installation (C; C1, C2) has at least one compressor wheel, with both wheels on a shaft, and that a motor is provided for supplying system-extraneous energy for the compressor installation.
Another object is that the compressor installation (C; C1, C2) and the expansion installation (T; T1, T2) are designed in such a way that the ratio of conveyed mass tapped airflow to conveyed mass ambient airflow is in the 100:0 to 50:50 range, and that the ratio of mass tapped airflow to mass ambient airflow is roughly 65:35.
A still further object of the invention is that a water separator (WE2) is arranged between the mixing element (xe2x80x9cXxe2x80x9d) and the expansion installation (T; T1), and that in a flow direction before the mixing element (xe2x80x9cXxe2x80x9d) a water separator (WE1; WE3) is provided for separating water from the compressed ambient airflow and/or the tapped airflow.
It is another object of the invention that the system is designed in such a way that a pressure of roughly 3.4 bar ensues for the mixed airflow at the mixing element (xe2x80x9cXxe2x80x9d), and that the tapped airflow and the ambient airflow are guided by a first and a second heat-exchanger (PHX/MHX) past a cooling airflow, in connection with which the first heat-exchanger (MHX) serves to cool the compressed ambient airflow and is arranged in the flow direction of the cooling airflow before the second heat-exchanger (PHX) to cool the tapped airflow.
A still further object of the invention is that a water injection device provided with which water separated from the tapped airflow and/or from the mixed airflow and/or from the compressed ambient airflow is guided, before the first heat-exchanger (PHX), into the cooling airflow for evaporation there, and that the expansion installation comprises two turbine stages (T1, T2) and the (first) heat-exchanger (CON) is arranged between the two turbine stages.
It is an object of the invention that a water separator (WE4) is arranged between the two turbine stages (T1, T2), and that the compressor comprises two compressor wheels (C1, C2) on separate shafts and each compressor wheel (C1, C2) is mounted, with only one turbine wheel in each case (T2/T1), on a shared shaft.
Another object of the invention is that one of the shafts in application with the preceding object is additionally equipped with a motor (M) that can also act as a generator (G), and that in the flow direction of the ambient airflow and behind the compressor installation (C1, C2), a surge valve (CSV) is provided in order to blow off largely uncompressed ambient air sucked in.
A further object of the invention is that a bypass valve (BPV1) is provided in order, during flight, to mix the compressed ambient airflow into the tapped airflow only after its pressure has dropped, and that a bypass valve (BPV2) is provided in order, during flight, to direct the tapped airflow past the mixing element (xe2x80x9cXxe2x80x9d) directly to the expansion installation (T; T1, T2)
For this purpose, it is provided for that only a portion of the fresh air to be prepared (bleed) is bled from the power unit under comparably high pressure. The other portion of the fresh air to be prepared is sucked from the ambient air, preferably as ram air, compressed and combined with the pressurized tapped airflow into a mixed airflow. The mixed airflow""s pressure is then dropped, preferably in one or more turbine stages. The energy gained during the expansion can be utilized regeneratively to compress the ambient airflow sucked in.
To solve the aforementioned technical problem, the invention furthermore provides for the tapped airflow to be guided in heat-exchanging manner past the mixed airflow in order to cool the tapped airflow before combining with the ambient airflow. This is essential for an effective water extraction from the tapped airflow. The efficacy of the water extraction results in particular from the fact that the cooler mixed airflow guided past the tapped airflow is comparably large compared to the tapped airflow, preferably at a ratio of 100:65.
It is particularly advantageous if the tapped airflow is guided in heat-exchanging manner twice past the mixed airflow, namely once before the mixed airflow""s pressure is dropped and another time after the mixed airflow""s pressure has been dropped.
The water is then preferably extracted from the mixed airflow before the mixed airflow""s pressure is dropped but may also or additionally be extracted already from the compressed ambient airflow or the cooled-off tapped airflow.
The air-conditioning system according to the invention with the highly effective water extraction from the tapped airflow makes it possible to use a single water extraction circuit and a conventional turbine arranged downstream from the water extraction circuit to drop the mixed airflow""s pressure. The system comprises roughly the same number of components as an air-conditioning system in which the entire cooling airflow to be prepared is bled from the power unit. The particular flow guiding of the tapped airflow and of the compressed ambient airflow by means of this system produces an air-conditioning system with effective water extraction that is highly efficient, in connection with which the power unit capacity is only affected a little and the system can be designed compact, not complex and with low weight, especially since no additional components are needed.
The air-conditioning system according to the invention thus has two circuits, a first circuit for the bleed and a second circuit for the ambient air, which are combined at a mixing point at which the same pressure, that is, the mean pressure, ensues for both circuits. This design requires that a change of parameters in the one circuit automatically affects the other circuit thus resulting in an overall system that is self-adjusting.
At the mixing point a mean pressure ensues that is between the pressure of the bled power unit air and the ambient air pressure or ram pressure. The bleed is fed to the system under a pressure in the 1.5 bar range in flight and 4 bar in ground operation, preferably 2 bar and 3.5 bar, respectively. The entire system is then preferably designed in such a way that at a mass tapped airflow/compressed mass ambient airflow ratio of approx. 65:35, a mixed air pressure of approx. 3.4 bar ensues at the mixing point in ground operation.
For the purpose of cooling off and subsequent water extraction, first of all the bleed as well as the compressed ambient air are each cooled off in a heat-exchanging process, for example in crosswise counterflow, with a cooling airflow from uncompressed and thus comparably cool ambient air. The heat-exchanger for cooling off the compressed ambient airflow and the heat-exchanger for cooling off the tapped airflow are advantageously connected in series in such a way that the cooling airflow from uncompressed ambient air can flow through them one after the other. In this way, the flow channel for the cooling airflow can be kept comparably narrow and be designed compactly, having a positive effect on the weight of the entire system. The heat-exchanger for cooling off the compressed ambient air is preferably arranged before the heat-exchanger for cooling off the bleed, to be able to cool off the compressed ambient airflowxe2x80x94making use of the maximal temperature dropxe2x80x94to such a low temperature that water from the compressed ambient airflow condenses.
The efficiency of the entire system can be optimized by appropriately designing the related heat-exchanger. A high level of efficacy of the heat-exchanger for the bleed is achieved by a high degree of concentration, in connection with which a considerable pressure loss via the heat-exchanger is acceptable because the bleed is bled from the power unit under comparably high pressure anyway.
The ratio of bled mass airflow from the power unit to compressed ambient air is high, preferably around 65 to 35. Due to the comparably small mass flow of the compressed ambient air, the heat-exchanger for cooling off the compressed ambient air can be designed very efficiently with low density, with a slight pressure loss occurring via this heat-exchanger. Efficiency is particularly high especially when the entire cooling airflow is used for cooling the relatively small ambient airflow (i.e., when the heat-exchangers are connected in series in the cooling airflow channel) and when, in addition, preferably the entire cooling capacity from the evaporation of the water obtained in the high pressure water extraction circuit is made available to this heat-exchanger.
A further design of the invention provides for the expansion of the mixed air to take place in two steps, and the condenser of the high pressure water extraction circuit is arranged between the two turbines. The system""s efficiency can be further improved by this measure. In this connection, water is advantageously extracted from the mixed air in an additional water extractor after the mixed air""s in the first turbine stage. This additional water extraction is not only advantageous in the air-conditioning system according to the invention as described here but rather in any system with two-step pressure expansion and condenser arranged in between.
Yet another design of the invention provides for not only the mixed airflow""s expansion but also the compression of the sucked-in ambient air to take place in two steps, with in each case a turbine wheel and a compressor wheel mounted on a shared shaft. Thus, altogether two shafts separate from each other and each with a turbine wheel and a compressor wheel are provided. In this way, a substantially more flexible design of the entire system and thus an even greater efficiency are achieved, particularly in flight.
In all of the aforementioned designs of the invention, a motor can advantageously also be provided on a shared shaft with the turbine wheel and the compressor wheel. This motor makes it possible to feed additional energy to the air-conditioning system at peak loads to generate an additional fresh air quantity and/or cooling capacity. In connection with the form of construction of the invention in which two shafts each with a turbine wheel and a compressor wheel are provided, this motor can also be used as a generator, particularly in flight.
These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.